Bridging wires have been used between resonator elements in mechanical filters for creating finite attenuation poles. In order to create poles on both sides of the upper and lower skirts of a filter, two or more resonators must be bridged with a coupling means such as a wire. If the length of the wire is close to a resonant length, the coupling will be sensitive to small variations in the length between attachment paints and spurious modes may be present in the filter response.
There is a considerable amount of prior art on the subject and a typical example of a prior art patent is Kasai, et al. U.S. Pat. No. 4,241,321, assigned to Fujitsu Limited, in Japan.
It is the extensional and flexural mode resonances which usually cause problems with coupling in bridging wires in a mechanical filter. Depending on what type of coupling is used, the effect of these two resonant modes are different. As will be explained in more detail later, at wire lengths commencing with zero and at various other lengths equal to half integer multiples of the resonant wavelength, the coupling increases to infinity. With relatively short wire lengths, where the order of the flexural spurious modes are low, the effect of the coupling of the modes is significant only when the wire length is within a few thousands of an inch of the resonant length. For higher order spurious modes, where the wire is longer, the effects of the flexural spurious modes are spread over a wider span of length. Because of this, it is advantageous to keep the wire short.
A solution to the problem as set forth in this invention, is to change the order of the resonators whereby, for several embodiments of the inventive concept, the bridging wire is required to cross only over one resonator. Prior art similar mechanical filters had to pass the bridging wire over at least two resonators. While the new design requires that the coupling wires for coupling signal from input to output to produce the bandpass filter must have some of the coupling elements increased in length, the overall design is still more tolerant of errors in attachment of the coupling wires used in the filter.
It is therefore an object of the present invention to improve the ease of manufacture of a mechanical filter.